What are these spots appearing along seams?
These look like jpg compression artifacts. jpg is a lossy format, which means that the saved image is not exactly what it was originally (the difference depends on the quality setting the file was saved with).

The advantage to lossy compression is that jpg files are typically very small compared to lossless formats. However it's not the best format to use during production, especially when a file is being edited and re-saved a lot. When saved in a lossy format repeatedly, the image will gradually lose quality. This process is called generation loss.

Try saving to a lossless format like png instead.